Drilling system fluids comprising water as the continuous phase often are ecologically favored over fluids in which oil is the continuous phase. Unfortunately, the walls of a wellbore frequently are composed, at least in part, of shale. When exposed to water, many shales swell, slough, or spall to the extent that they prevent further operation of the wellbore. Shale also may slough during gravel transport in open-hole completion, mix with the gravel, and/or reduce the productivity of the well by choking off the permeability of the gravel pack. Shale sloughing also may cause screen blinding.
The use of invert emulsion drilling system fluids may help to minimize shale hydration. An invert emulsion fluid comprises “oil” as the continuous phase and an emulsified aqueous dispersed phase. In order to minimize hydration of shale, the aqueous dispersed phase typically comprises a brine comprising metallic salts at a salinity greater than the salinity of the interstitial water in the shale.
Unfortunately, contamination by highly reactive drilled solids may cause rheological and/or filtration control problems and also may decrease emulsion stability in invert emulsion drilling system fluids. Alternative invert emulsion drilling system fluids are needed.